In recent years, in the fields, such as a car and a flight object, in order to detect obstacles existing forward in a travelling direction, for example, a light scanning type object detecting device that emits a laser beam while scanning, receives reflected light that hits an object and is reflected from it, and obtains information regarding the obstacles on the basis of a time difference between the time of emitting light and the time of receiving the light, has been developed and already put in practical use.
Such an object detecting device can be applied, in addition to the above-mentioned obstacle detection for a movable body, to crime prevention use for detecting suspicious person by disposing it under the eaves of buildings, geographical feature investigation use for acquiring geographical feature information from the sky by mounting it in a helicopter, an airplane, etc., and gas detection use for measuring the gas concentration in atmospheric air.
In a general light scanning type object detecting device, a light projecting system is constituted by laser diodes as a light source and a collimating lens, a light receiving system is constituted by a light receiving lens (or mirror) and a photodetector, such as photodiode, and, further, a reflective mirror provided with a reflective surface is disposed between the light projecting system and the light receiving system. In such a laser scanning type object detecting device, light emitted from a light projecting system is projected so as to perform scanning by the rotation of a reflective mirror, whereby there are merits that not only measurement is performed for one point, but also measurement can be performed two dimensionally for an object in a wide range. In this connection, as a light source, in addition to the laser, LED etc. may be used.
In the case of taking a laser light source for an example, as a scanning method for a general laser light flux, a technique has been known that projects a laser light flux onto a mirror or a polygon mirror with a plurality of mirror surfaces, and oscillates the mirror or rotates the polygon mirror, whereby the laser light flux is made to scan.
Patent Literature 1 discloses a constitution in which a first reflective surface and a second reflective surface are formed with a nipping angle of 90 degrees between them on a rotation mirror, and a light flux is emitted from a light source along a direction orthogonal to the rotation axis, is reflected two times on the first reflective surface and the second reflective surface, and then performs scanning, whereby the disorder of a scanning line is not caused even if a rotation axis inclines due to rotation fluctuation. Moreover, Patent Literature 2 discloses a laser radar in which a plurality of pairs of first mirror and the second mirror are disposed, and an intersecting angle between the first mirror and the second mirror is changed for each of the plurality of pairs, whereby scanning can be performed for a plurality of different sub-scanning positions by one rotation.